Initial stages of erbium disilicide formation on Si(0 0 1) Jianshu Yang a,b, * , Qun Cai a , X.-D. Wang a,1 , R. Koch b a Surface Physics Laboratory (State Key Laboratory), Fudan University, Shanghai 200433, China b Paul-Drude-Institut f€ ur Festk€ orperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany Received 22 June 2002; accepted for publication 4 November 2002 Abstract The initial stages of ErSi 2 formation on Si(0 0 1) are investigated by scanning tunneling microscopy and low energy electron diffraction. At sub-monolayer coverages two Er-induced superstructures are observed, where Er dimers are replacing the Si dimers. In the cð4  2Þ phase Er dimers alternate with Si dimers both along and perpendicular to the Si dimer row direction. The (2  3) phase, which evolves naturally from the cð4  2Þ phase with increasing coverage, consists of two adjacent chains of Er dimers and can be regarded as a precursor of the strain-field-controlled ErSi 2 nanowire growth. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Surface structure, morphology, roughness, and topography; Lanthanides; Silicon 1. Introduction Erbium disilicide (ErSi 2 ) has attracted consider- able attention in the recent years as being a promising candidate for fabrication of low resis- tance ohmic contacts in Si technology as well as of wide area infrared detectors in micron and sub- micron C-MOS technology [1,2]. It is metallic with a high electrical conductivity of 2:9  10 4 X cm and exhibits an extremely low Schottky barrier against n-type Si (0.28 eV) [1,3–6]. Epitaxial ErSi 2 thin films on Si surfaces are obtained by thermal reaction between Er and Si at 400–900 °C [7–12]. Common preparation proce- dures are: (i) Er deposition followed by annealing at 400–700 °C, (ii) deposition of Er directly at el- evated temperatures of 600 °C and (iii) Er and Si co-deposition at room temperature followed by annealing at temperatures of 700–900 °C. In its bulk form ErSi 2 crystallizes in the AlB 2 crystal structure––analogous to most of the rare-earth silicides––where along the [0 0 0 1] direction hexa- gonal Er planes alternate with planes of Si atoms in a honeycomb arrangement. The primitive unit cell parameters a and c are 0.379 and 0.409 nm, respectively. Owing to its threefold symmetry and the good matching of the lattice parameters (a Si–Si ¼ 0:384 nm), Si(1 1 1) has become the pre- ferred substrate for ErSi 2 heteroepitaxy; the tensile misfit strain is only þ1.3%, which explains the high quality epitaxial growth. Though being the tech- nologically more interesting substrate, only few studies were performed so far on Si(0 0 1) which * Corresponding author. Address: Paul-Drude-Institut f€ ur Festk€ orperelektronik, Hausvogteiplatz 5-7, D-10117 Berlin, Germany. Tel.: +49-3020377393; fax: +49-3020377257. E-mail address: yang@pdi-berlin.de (J. Yang). 1 Present address: Department of Physics, the University of Texas, Austin, TX 78712, USA. 0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0039-6028(02)02549-9 Surface Science 526 (2003) 291–296 www.elsevier.com/locate/susc